1. Field of the Invention
The present invention relates to an alkaline fuel cell using an anion conductive electrolyte membrane (anion exchange membrane) as an electrolyte membrane, and an alkaline fuel cell system which can adjust a temperature of an alkaline fuel cell using a heat medium.
2. Description of the Background Art
Fuel cells have a possibility of achieving reduction in size and weight and high output density, and development of their application to a new power source for mobile electronic devices, to household cogeneration systems, and the like is aggressively promoted. A fuel cell includes, as a main portion for power generation, a membrane electrode assembly (MEA) formed by sandwiching an electrolyte membrane between an anode electrode and a cathode electrode. Depending on the type of the electrolyte membrane, fuel cells are classified into a solid polymer fuel cell (including a direct fuel cell), a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, an alkaline fuel cell, and the like.
An alkaline fuel cell is a fuel cell which uses an anion conductive electrolyte membrane (anion exchange membrane) as an electrolyte membrane and in which a hydroxide ion (OW) serves as a charge carrier. In the alkaline fuel cell, when an anode electrode and a cathode electrode are electrically connected, a current flows between the anode electrode and the cathode electrode by an electrochemical reaction described below, and thus electric energy can be obtained. Specifically, when an oxidant (for example, oxygen, air, or the like) and water are supplied to the cathode electrode, OH− is generated by a catalytic reaction represented by the following formula (1):cathode electrode: 1/2O2+H2O+2e−→2OH−  (1).This OH− is transferred to the anode electrode through the electrolyte membrane, in a state hydrated with a water molecule. On the other hand, at the anode electrode, a supplied fuel (reductant), for example, H2 gas, and OH− transferred from the cathode electrode cause a catalytic reaction represented by the following formula (2) to generate water and electrons:anode electrode: H2+2OH−→2H2O+2e−  (2).
Since an anion conductive electrolyte is used as the electrolyte membrane and an electrolyte of a catalyst layer in the alkaline fuel cell, the electrolyte membrane and the catalyst layer absorb carbon dioxide (CO2) in an atmosphere during suspension of operation, and OH− in the electrolyte membrane and the catalyst layer is substituted by CO32− and/or HCO3− (hereinafter may be referred to as a “CO2-derived anion”) by a reaction represented by the following formulas (3) and (4):CO2+2OH−→CO32−+H2O  (3)CO2+OH−→HCO3−  (4).An increase in the concentration of such CO2-derived anions (a decrease in OH− ion concentration) decreases anion conductivity of the electrolyte and significantly increases cell resistance.
It is known that the above problem of the increase in cell resistance can be improved by a phenomenon called self purge caused by operating the fuel cell. The self purge refers to a phenomenon in which, due to operation of the fuel cell, CO2-derived anions contained in the electrolyte membrane and the catalyst layer and serving as a factor of the decrease in anion conductivity move to the anode electrode, are reduced by the fuel, and emitted as CO2 gas from the anode electrode. The self purge can be specifically represented by the following formulas (5) and (6):H2+CO32−→CO2+H2O+2e−  (5)H2+2HCO3−→2CO2+2H2O+2e−  (6).
However, as described in ECS Transactions, 25(13), 105-110 (2010) by Yu Matsui, Morihiro Saito, Akimasa Tasaka, and Minoru Inaba [Non-Patent Literature 1], although the self purge can suppress an increase in cell resistance because many CO2-derived anions are resubstituted by OH−, CO2-derived anions remaining in a certain amount are localized by the self purge at the anode electrode and accumulated therein, which causes a problem that reaction overpotential at the anode electrode is increased and power generation efficiency is decreased.
Japanese Patent Laying-Open No. 3-295175 [Patent Literature 1] describes removing carbon dioxide in a fuel supplied to an anode electrode by bringing the fuel into contact with an alkaline aqueous solution. Further, “Development of Anion Exchange Membrane Fuel Cells” by Yotaro Yamazaki, the Ministry of Education, Culture, Sports, Science and Technology Science Research Grant “New Development in Highly Efficient Energy Conversion with Low Environmental Load by DMFC” (Grant-in-Aid for Scientific Research on Priority Area B) Research Report, pp. 71-74, June 2006 [Non-Patent Literature 2] describes improving output characteristics by adding alkali beforehand to a fuel to be supplied to an anode electrode.
As indicated in the above formula (1), in the alkaline fuel cell, it is necessary to supply water to the cathode electrode for the catalytic reaction. In addition, it is also necessary to supply water to the electrolyte membrane to prevent drying of the electrolyte membrane and resultant increase in an anion conduction resistance. Conventionally, such water supply is generally performed by using a humidified fuel and/or oxidant as a fuel to be supplied to the anode electrode and/or an oxidant to be supplied to the cathode electrode. However, this method requires auxiliary equipment such as a humidifier, and results in a larger fuel cell.
International Publication No. 2009/149195 [Patent Literature 2] describes directly supplying liquid water (i.e., water in the form of a liquid) to a surface of an electrolyte membrane on a side facing a cathode electrode, as another water supply method (see, for example, page 28, line 18 to page 31, line 18, FIGS. 11 and 12). More specifically, in this method, an outer edge portion of a cathode-side electrode portion in a gasket is provided with a slot directly connected to the surface of the electrolyte membrane on the side facing the cathode electrode, and liquid water is directly supplied from the slot to the surface of the electrolyte membrane on the side facing the cathode electrode (page 31, lines 4 to 18, FIG. 11).
Meanwhile, fuel cells such as an alkaline fuel cell are generally adjusted in an appropriate temperature range during power generation, considering increase in the efficiency of the above catalytic reaction (electrochemical reaction) and prevention of thermal deterioration of fuel cell components such as an electrolyte membrane. One of conventional examples of a temperature adjustment method is to place a heater in a fuel cell. However, since temperature adjustment by the heater cannot perform cooling, it is inevitable that the fuel cell has a high temperature, particularly when the fuel cell generates a large amount of heat, such as when a large current is drawn from the fuel cell.
“Technology for Fuel Cells: Problems and Countermeasures of Solid Polymer Type” written by Hisao Nishikawa, Tokyo Denki University Press, 2010, pp. 45-46 [Non-Patent Literature 3] describes passing cooling water through a flow channel provided inside a separator, as a method for cooling a fuel cell (page 46, FIG. 3.22). It is noted that the wording “provided inside a separator” used herein means that the flow channel is not open to a membrane electrode assembly, and is provided inside the separator in a state separated from the membrane electrode assembly.